In general, naphthalenedicarboxylic acids are produced by a method in which substituted naphthalenes having two substituents each capable of being converted into a carboxyl group through oxidation reaction, e.g., di-alkyl-substituted naphthalenes such as dimethylnaphthalenes and diisopropylnaphthalenes, are oxidized with molecular-state oxygen in the presence of cobalt, manganese, and bromine. Since the crude naphthalenedicarboxylic acids obtained by this method contain impurities, e.g., trimellitic acid, and coloring substances, a purifying step is necessary.
A conventionally known purification method for naphthalenedicarboxylic acids comprises dissolving a crude naphthalenedicarboxylic acid in an aqueous solution of an alkali, subjecting the solution to such treatments as oxidation, hydrogenation, decoloring by adsorption, etc., and then acidifying the resulting solution, thereby to obtain the naphthalenedicarboxylic acid having a high purity (JP-A-48-68554, JP-B-52-20993, JP-A-50-105639, JP-A-50-160248and others). (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively.)
However, the above method has had a problem that because an alkali and an acid are used in large quantities, an inorganic salt is produced and wastewater is discharged in large quantities.
On the other hand, JP-A-62-230747 discloses a purification method which comprises dissolving a crude naphthalenedicarboxylic acid in an organic solvent selected from N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO), treating the solution with active carbon, and then recrystallizing the naphthalenedicarboxylic acid.
The above method, however, has a drawback that due to the low solubilities of naphthalenedicarboxylic acids in DMF or DMSO, the solvent should be used in a large amount. Further, it has been found, from purification experiments conducted by the present inventors according to the above method, that when a purified naphthalenedicarboxylic acid is obtained in such a manner as to result in a higher recovery, almost no improvement is attained in the color of the compound (see Comparative Examples 6 and 7). In addition, the above method has had a further problem that those organic solvents are difficult to recover because of their high boiling points and are highly toxic.
JP-A-50-142542 discloses a purification method which comprises dissolving crude 2,6-naphthalenedicarboxylic acid in an aqueous solution of a specific alkylamine, e.g., dimethylamine, and then removing the amine from the solution by distillation thereby to precipitate the 2,6-naphthalenedicarboxylic acid.
However, the above purification method has had drawbacks such as the following: (1) amines usable in the method are limited to those which can be easily distilled, because in order to precipitate the naphthalenedicarboxylic acid, the amines should be removed and recovered by distillation; (2) a large proportion of the water in the aqueous solution is lost along with the amine since the amine evaporates as a boiling mixture with water; and (3) the recovery attainable by the method is low because complete removal of the amine from the aqueous solution is impossible.